In surgical procedures involving the spine, it is common to secure a pair of spinal rods to a series of vertebrae so that the rods are aligned essentially parallel to the spinal column. The spinal rods may serve to immobilize vertebrae, preventing unwanted flexion, extension, and rotation of vertebrae with respect to each other. It is often further desirable, or even necessary, to provide a connecting device to extend transversely between spinal rods, thereby securing the spinal rods relative to each other.
Spinal rods are typically anchored to the vertebrae by a series of anchor members in the form of bone screws that extend through the pedicles of vertebrae or hooks that engage the vertebrae. The spinal rods are connected to the screws or other anchor members by coupling members or yokes designed to receive and secure a rod. For instance, the coupling member may form a u-shaped channel that is closed with a set screw or other cap device that locks the spinal rod in place within the channel. Such coupling members may be integral with the anchor member, or may be provided as a separate component in order to allow for polyaxial movement of the anchor member.
When utilizing crosslinking or connecting devices to secure pedicle screws, a number of obstacles are commonly encountered. Spinal rods are mounted by a surgeon in a custom-fit manner, including some bending of the rod, so that the rod extends properly along the spine for holding the vertebral portions in proper relation. Accordingly, there is often not a constant distance between two spinal rods, and instead the rods may converge or diverge from each other. In addition, one spinal rod may have a portion that extends transversely and/or is curved relative to the other rod so that the central axes of neighboring rod segments are non-parallel.
One attempt at overcoming these obstacles and to facilitate linking of non-parallel portions of spinal rods is a crosslinking system that includes two opposed ends comprising clamping devices for securing the spinal rods, with the clamping devices linked to each other by a central cross rod or rods, typically allowing adjustment of the distance between the connector ends by axial movement of the cross rod, pivotal adjustment of one connecting device relative to the cross rod, and rotation of the connecting devices relative to each other about the axis of the cross rod. However, providing multiple points of articulation presents further problems. For instance, the components of the cross connector must be configured in such a way that they do not interfere with each other when arranged in various configurations. Furthermore, each articulation point must be capable of locking in order to secure the spinal rods in a desired configuration, requiring multiple locking mechanisms that must be loosened and tightened in order to adjust and secure the cross connector. Additionally, many devices include a locking mechanism in the center of the crosslinking device to prevent translation, pivoting, and or rotation of the cross rod, which can be difficult for surgeons to reach and lies close to the spinal cord when implanted.
U.S. Pat. No. 6,875,211 discloses a device for connecting two spinal rods to one another that includes deformable clamp bodies at opposite ends of the device in which a cam lug is rotated to move opposed clamp arms to a clamped position. The distance between the clamp bodies can be adjusted by shifting a shaft connected to one clamp body within a bore of a body portion connected to the other clamp body. The position of the shaft within the bore may be locked by a radially compressing ring or by providing threads on the shaft and bore so that rotation of the clamp bodies relative to one another adjusts the length of the device. These features for adjusting the length of the clamp body are not easy to manipulate, and limit the positions at which the length may be locked. Furthermore, the device does not allow for pivoting the clamp bodies relative to one another, making it difficult to use the device to connect spinal rod segments that are skewed relative to one another. Additional locking mechanisms would be needed in order to provide the device with additional degrees of adjustability.
Published U.S. Patent Application 2005/0228377 discloses a device having a pair of connector members with first and second jaws that are pulled together by a threaded member in order to engage a spinal rod. The device relies on the threaded member to maintain the jaws in the pulled-together position, and loosening of the threaded member would lead to release of the spinal rod. Additionally, all adjustment of the positioning of opposed connector members relative to one another is accomplished by a central joint that permits translation and pivoting in a single plane. A central clamp locks the positions of the connector members relative to one another. Thus, the device cannot easily be adjusted to accommodate bent or skewed spinal rods.
Published U.S. Patent Application 2006/0271045 discloses a device having a pair of connector members that each have a separate jaw portion secured to the connector member by a threaded screw. Tightening of the screw locks the spinal rod within the connector member. Loosening of the screw, however, would allow the jaw portion to fall away from the connector member and release the spinal rod. All adjustment of the positioning of the connector members relative to one another is performed by a central clamp mechanism, which allows translation and pivoting of the connector members in a single plane. Thus, the device cannot easily be adjusted to accommodate bent or skewed spinal rods.
Therefore, there remains a need for devices for connecting spinal rods that provide for multiple degrees of adjustability and secure locking of the spinal rods.
Rigid or semi-rigid elongate members, such as spinal rods, may be mounted to the spinal column in order to stabilize or immobilize vertebrae of the spinal column for a variety of purposes. For instance, spinal rods are often secured to adjacent vertebral bodies via anchor members in order to promote fusion of the two vertebrae as a treatment for degenerative disc disease, spondylolisthesis, spinal stenosis, fractures of the vertebrae, and other conditions. Limiting or preventing motion of the vertebrae promotes the healing process. By removal of the disc positioned between the vertebrae and limiting motion between the vertebrae, the adjacent boney surfaces are allowed to grow into one another and fuse together. Fusion devices may also be placed between the two immobilized vertebrae in order to facilitate the process of fusion.
When stabilizing portions of the spinal column, and in particular the cervical region of the spine, it is sometimes necessary to immobilize the skull in addition to vertebrae. The same elongate rigid structures used to link and stabilize the vertebrae may therefore be secured to the skull in order to keep the skull in an appropriate spatial relationship with respect to the spinal column. However, since the anatomy and thickness of the skull and its surrounding tissues are very different than those of the vertebrae and their surrounding tissues, the elongate rigid structures must be anchored to the skull in a different manner than that used for the vertebrae.
For instance, in many spinal stabilization procedures elongate rods made of titanium or other materials are placed adjacent to the posterior side of the spine and anchored in place using screws connected to some type of coupling assembly. Examples of suitable coupling assemblies for posterior fixation systems are disclosed in U.S. Pat. No. 7,141,051; U.S. Published Application No. 2008/0045955; and U.S. Published Application No. 2007/0225711. The screws used to anchor these devices and other coupling assemblies are often relatively long, and are mounted to the pedicle area of the vertebrae with the shanks of the screws penetrating deep into the vertebral body. The yoke portion of the coupling assembly that is coupled to the screw and receives the spinal rod is nested between outwardly-extending boney processes so that the height of the yoke is not noticeable.
When spinal rods are mounted to the skull, however, long screws and large coupling assemblies cannot be mounted directly to the skull without undue risk of penetrating the brain encased therein. Many coupling assemblies also would prove extremely and unduly cumbersome if mounted directly to the skull, and may even protrude significantly from the back of the head. In addition, the occipital region, which juts out at the base of the skull, is the only reasonable site at which to mount an internal fixation system, requiring that spinal rods connected thereto be bent severely in order to be positioned along the occipital region and be connected to the occipital region in a manner similar to the connection to the vertebrae.
Previous systems for coupling spinal rods and other elongate stabilization devices to the skull vary. However, most systems utilize a plate mounted to the occipital region of the skull that attaches to a rod, cable, wire, plate, or screw mounted to a region of the spine. In most spinal rod systems, two spinal rods are positioned generally parallel to the surface of the plate and then secured thereto by a bracket or u-shaped receiving member. The plates are mounted to the skull with several small screws disposed along the full length and width of the plate. Since the base of the skull angles inward toward the spine, the plates mounted to the skull are not parallel to the posterior surfaces of the vertebrae, and the spinal rods must be bent significantly away from the vertebrae in order to reach the occipital region in an orientation that may be mounted to the plate. For instance, the bending of spinal rods in order for them to properly be received relative to an occipital plate is shown in the devices of FIGS. 1, 2, and 18 of U.S. Published Application No. 2004/0153070. In that device, spinal rods mounted along the vertebrae must be manipulated in order to fit precisely into receiver mechanisms aligned along the sides of a plate designed to be fixed to the occipital region of the skull. This bending of the rod can fatigue the rod material, and also makes it difficult to reposition the elements of the stabilization system.
Even attempts to provide occipital plate devices with adjustability in order to accommodate spinal rods of various orientations still generally require significant manipulation and bending of spinal rods before they can be secured to the plate structure. For instance, U.S. Pat. No. 6,902,565 discloses a plate designed to be mounted to the occipital region of the skull by a plurality of short expansion head screws. The plate receives a pair of rods that may be further mounted to one or more vertebrae. In many cases these rods are pre-bent so that the majority of the rods may be positioned parallel to the spine, with the ends bent transversely in order to be secured to the plate by a clamp plate or bracket. Some embodiments include plates that are bent in order to receive the rods that are parallel to the spine. However, in all cases the devices provided to clamp the rods in place on the plate are designed to accept the rod in only one position, so that any variation in angle or spacing of the rods caused by the patient's anatomy requires bending of the spinal rods in order to properly secure them to the plate.
U.S. Published Application No. 2008/0051783 discloses a plate device having a pair of u-shaped rod receiving members that protrude from lateral wings of the plate. The wings may be shifted laterally and medially, and the rod receiving members may rotate to adjust the direction in which a connecting member (such as a spinal rod) is received. Therefore, the device does allow some adjustability in order to receive the spinal rods. However, the spinal rods must be positioned so that they are generally parallel to the plate surface in order to fit into the rod receiving members. Therefore, the ends of the rod must be bent away from the axis of the spine and into the u-shaped channels of the receiving members.
U.S. Pat. No. 6,524,315 discloses a plate secured to the bone by a plurality of screws. The plate is fitted with slotted bolts designed for receiving a rod or cable. The base of the slotted bolt is recessed in the plate at its base. A support platform may be fitted over the bolt to help hold the rod or cable. A nut fastens over the threaded end of the slotted bolt to trap the rod or cable within the bolt, securing it to the plate. While the bolt may be rotated to adjust the direction of the rod or cable, this adjustment affects only one plane, and does not allow for adjustment of the angle of the rod or cable with respect to the surface of the plate.
U.S. Published Application 2007/0233119 discloses a plate device with polyaxial connector head assemblies including a connector body that receives a spinal rod and a connector head pivotably connected to the connector body and configured to be secured to the plate so that the connector assemblies provide limited polyaxial movement of the spinal rods with respect to the plate. However, the device does not allow multiple types of movement to provide a highly articulated device. Furthermore, the coupling heads are relatively bulky and still hold the spinal rods relatively parallel to the plate surface.
Therefore, improved devices for securing spinal rods to the occipital region of the skull are desirable.